He became an anti-imperialist after hearing about the atrocities from the US in the phillipines:

I left these shores, at Vancouver, a red-hot imperialist. I wanted the American eagle to go screaming into the Pacific. It seemed tiresome and tame for it to content itself with he Rockies. Why not spread its wings over the Phillippines, I asked myself? And I thought it would be a real good thing to do

I said to myself, here are a people who have suffered for three centuries. We can make them as free as ourselves, give them a government and country of their own, put a miniature of the American constitution afloat in the Pacific, start a brand new republic to take its place among the free nations of the world. It seemed to me a great task to which had addressed ourselves.

But I have thought some more, since then, and I have read carefully the treaty of Paris, and I have seen that we do not intend to free, but to subjugate the people of the Phillippines. We have gone there to conquer, not to redeem. . .

It should, it seems to me, be our pleasure and duty to make those people free, and let them deal with their own domestic questions in their own way. And so I am an anti-imperialist. I am opposed to having the eagle put its talons on any other land.